1. Field of the Invention
The present invention generally relates to a wavelength tunable optical filter that adjusts a distance between two mirrors to make a desired wavelength of light incident or output and, more specifically, to a Fabry-Perot type wavelength tunable optical filter.
2. Discussion of Related Art
In general, a driving method of a wavelength tunable optical filter is largely classified into two categories. One is to adjust a distance between mirrors by a force applied to the mirrors and to provide a restoration force by a structure connected to the mirror as in an electrostatic scheme (C. J. Chang-Hasnain, IEEE J Select. Topics Quantum Electron., vol. 4, pp. 978–987, 2000), and the other is by a deformation of the driving body that is connected to the mirror as in a thermal expansion scheme (C. K. Kim et al., Proceeding of IEEE/LEOS Optical MEMES Conference 2003, pp. 38–39, 2003), an electromagnetic scheme (C. K. Kim, Korean Patent Application No. 10-2003-0029763), and an external mechanical force scheme (N. Yokouchi et al., IEEE Photon. Technol. Lett., vol. 4, pp. 701–703, 1992).
A wavelength tuning range of the wavelength tunable optical filter is determined by the maximum displacement of the driving body. For the electrostatic driving scheme to which attraction between two mirrors is applied, when the distance between two mirrors is reduced to less than ⅔ of the initial gap, a sticking phenomenon occurs, thus restricting the driving range to the ⅓ of the initial gap. To overcome this, a structure driven in a direction that the distance between the mirrors becomes farther is introduced. One method for this is that one fixed mirror, one moving mirror combined with an electrode, and a fixed electrode are arranged one after another, and a distance between the moving mirror and the fixed electrode is made larger than the distance between two mirrors, thus enabling a larger wavelength tuning range (D. C. Flanders, U.S. Pat. No. 6,341,039), and another method is that a lever type in which a mirror and an electrode unit are placed to be isolated from each other at both sides of a long rod structure is employed to increase a driving range as well as not to incur the sticking phenomenon (C. F. R. Mateus et al., IEEE Photon. Technol. Lett., vol. 14, pp. 819–821, 2002).
The structure in which the rod type driving body is deformed by means of the thermal expansion, the electromagnetic force or the external mechanical force also employs the physical deformation of the driving body only, thus limiting the displacement of the mirror.